For example, Japanese unexamined patent publication 2006-242907, which corresponds to US patent application publication 2006/0164254 A1, discloses an electrostatic occupant detection apparatus. Such an electrostatic occupant detection apparatus includes an electrostatic sensor having a main electrode, a sub-electrode, and a guard electrode. The main electrode is located in a seat portion of a vehicle. The sub-electrode is spaced from the main electrode. The guard electrode is located between a vehicle body, which is conductive with a vehicle grounding portion, and the main electrode and spaced from the vehicle body and the vehicle grounding portion. The electrostatic sensor sets the main electrode and the guard electrode to be in the same potential to form an electric field between the main electrode and the vehicle body so as to detect an electric capacity of an occupant on a seat portion and the like. The electrostatic sensor outputs an electric current or a voltage corresponding to change in a weak electric field caused between the main electrode and the vehicle body. An occupant detection electronic control unit (ECU) recognizes an occupant according to the outputted electric current or the voltage value.
In such a manner, an electrostatic sensor detects an electric capacity of an occupant and the like. However, when the electrostatic sensor is out of order, the electrostatic sensor may make an erroneous determination of an occupant. In view of such a problem, U.S. patent application publication 2006/0164254 A1 proposes an electric capacitance occupant sensor configured to self-detect a failure of the electrostatic sensor. The electric capacitance occupant sensor includes an occupant detection electrode for detecting an occupant being seated and a capacitance reduction electrode opposed to the occupant detection electrode. The capacitance reduction electrode reduces an electric capacity between the occupant detection electrode when an occupant is not seated and a seat frame from an electric capacity between the occupant detection electrode when an occupant is seated and the seat frame. The seat frame is electrically conducted with a vehicle grand (GND). When detecting a failure, the electric capacitance occupant sensor applies a voltage between the occupant detection electrode and the capacitance reduction electrode to detect an electric capacity between the occupant detection electrode and the capacitance reduction electrode. In the present detection, when a wiring of the occupant dejection electrode or the capacitance reduction electrode is disconnected, the electric capacitance occupant sensor is in a non-detection state of an electric capacity. In this manner, a failure of the electric capacitance occupant sensor is detectable when an occupant is not seated.
The occupant detection electrostatic sensor of US patent application publication 2006/0164254 A1 is capable of self-detecting a failure of the sensor when a wiring of the occupant detection electrode or the capacitance reduction electrode is disconnected to cause an open failure. Nevertheless, the occupant detection electrostatic sensor cannot determine a short failure caused by short-circuit between the wiring and another electric conduction portion. It is significantly important to detect a failure of an electrostatic sensor for detecting an electric capacity of an occupant. In particular, an electrostatic sensor may include a main electrode, a sub-electrode, and a guard electrode. In such a case, an open failure and a short failure of each of the electrodes need to be detected.